The invention relates to a protective barrier of the kind having a net installed across a slope to protect property and persons from falling rocks and avalanches.
The invention relates more particularly to the structure of the supports that hold the net in position across the slope.
In the mountains, the two major hazards are rock falls and avalanches. Some places at risk can be equipped with structures including very strong metal nets installed across the slope and held in position by a plurality of supports anchored to the ground. The imperatives are different according to the nature of the hazard: rock falls or avalanches.
Barriers protecting against rock falls are designed to intercept falling rocks and to dissipate their kinetic energy. This is achieved by deformation of the net, which usually has expandable meshes. The net is held in position by a plurality of supports spaced along a contour line of the slope. These structures are subjected to concentrated and dynamic loads. Barriers protecting against rock falls are covered by French standard NF P 95-308 which defines the specifications of the structures and classifies their performance.
The nets are conventionally supported by posts positioned normal to the slope and stayed in the uphill, downhill and lateral directions. The conventional system has the drawback that the supports are extremely vulnerable. Moreover, it is necessary to provide the same number of ground anchor points as there are stays. This makes installation on site costly.
Published French patent No. 2 622 611 describes a structure in which the supports are downhill of the net and consequently protected from impact by the net itself. The number of ground anchor points is nevertheless large.
French patent No. 2 712 334 describes a support having a post connected to a wide base (generally a tripod) to the periphery of which the bottom ends of the stays are fixed. This reduces the number of anchorages to one per support, but this system requires a heavy base of high inertia that can resist bending stresses.
On the other hand, barriers protecting against avalanches must be designed not to stop an avalanche but to oppose movements of the mantle of snow that lead to avalanches.
In other words, the net must oppose the thrust of the snow; it is subjected to a uniform and static load. In this type of structure, the net is inclined in the downslope or downhill direction, for example at 30° to the normal to the slope. The supports are spaced along a contour line. There is a post every five meters, for example. Barriers protecting against avalanches are covered by French standard NF P 95-304 which defines the dimensional requirements and the strength of various types of structure.
Avalanche nets are conventionally supported by stayed posts that are inclined in the upslope or uphill direction to the normal to the ground. Adapting the structure described in patent No. 2 712 334 to provide protection against avalanches by modifying the inclination of the posts to the normal to the slope and modifying the base to provide a bipod structure have also been envisaged.
Improvement and enlargement of winter sports resorts are leading to the provision of protection against rock falls and snowslides on slopes very close to heavily populated places. There is therefore a need for structures that are more discreet and that integrate more readily into an urban mountain landscape. The proliferation of places to be protected is also leading to the design of structures that are less costly to fabricate and easier to install. From the esthetic point of view, bulky steel structures are less and less tolerable for this kind of application.
Finally, it is desirable for the same structural members to be usable to build either a barrier to protect against rock falls or an avalanche barrier and for the materials used to harmonize as much as possible with the environment.